docs: def com_adobe_fonts_check_find_empty_letters(ttFont):
      """Letters in font have glyphs that are not empty?"""
      cmap = ttFont.getBestCmap()
      passed = True
  
      # http://unicode.org/reports/tr44/#General_Category_Values
      letter_categories = {
          'Ll', 'Lm', 'Lo', 'Lt', 'Lu',
      }
      invisible_letters = {
          0x115F, 0x1160, 0x3164, 0xFFA0,  # Hangul filler chars (category='Lo')
      }
      for unicode_val, glyph_name in cmap.items():
          category = unicodedata.category(chr(unicode_val))
          if (_quick_and_dirty_glyph_is_empty(ttFont, glyph_name)) \
                  and (category in letter_categories) \
                  and (unicode_val not in invisible_letters):
              yield FAIL, \
                  ("U+%04X should be visible, but its glyph ('%s') is empty."
                   % (unicode_val, glyph_name))
              passed = False
      if passed:
          yield PASS, "No empty glyphs for letters found."